PROJECT SUMMARY (See instructions); We have observed significant enhancement of islet engraftment, prolongation of islet allograft survival and reversal of rejection in cynomolgus monkey recipients of intrahepatic islet/MSC cotransplants. Our goal is to identify a safe and optimally effective transplant protocol that can be tested in the clinical setting to improve long-term islet allograft survival and function. MSC are being utilized experimentally and clinically to mediate inflammation and immunity in a variety of settings; however, the impact of MSC MHC in relation to the recipient and to the cellular or solid organ donor on transplant outcomes has not been defined. In the setting of islet allotransplantation, MHC matching is not taken into consideration when pairing a donor with a recipient. As MSC expansion and banking can take up to 6 weeks, it would be impractical in the clinical setting to utilize MSC from the islet donor. The alternatives are recipient MSC, which would require bone marrow aspiration and MSC expansion/banking while the recipient is on the waiting list, or MSC from a third party. The effect of MHC on islet/MSC transplant outcomes will be studied in Aim 1: To utilize a cynomolgus monkey, intrahepatic islet/MSC co-transplant model to identify the optimal source of the MSC product, i.e., recipient or 3rd party, by determining how MSC origin impacts islet allograft outcome. Our preliminary data suggests that IV administration of additional MSC at the time of islet allograft destabilization allows for reversal of rejection and, ultimately, enhancement of graft function. This will be assessed in Aim 2: To determine if intravenous administration of the optimal MSC product can reproducibly lead to reversal of islet allograft rejection and subsequent maintenance of or improvement in islet function. Based on published data regarding the mechanisms responsible for the immunomodulatory effect of MSC, as well as on our own preliminary data (Projects 1 and 2), we hypothesize that transplantation of MSC into the liver with islets, as well as infusion of IV MSC post-transplant will result in the induction of T regulatory cells, recruitment of regulatory monocytes/macrophages and endothelial precursors to sites of inflammation (i.e., the graft site) and migration of MSC to lymph nodes draining the graft site. We will undertake studies to address this in Aim 3: To define predictive biomarkers of MSC efficacy and non-efficacy in renal and islet allograft responses. Finally, the data from Projects 1 and 2 and Cores B and C will be incorporated in Aim 4: To undertake transplants with the optimal MSC source, dose and timing of administration, coupled with prospective monitoring of potential biomarkers, in order to enable a pilot clinical trial of islet/MSC cotransplantation.